In mining operation a driven element or bit is used to dislodge the bedded material such as for example coal. Because of the forces on the bit and the abrasiveness of its contact with the material, bits are subject to high wear and must be replaced at periodic intervals in the mining operation. It is therefore desirable to have a means for attaching the bit to the mining equipment that is secure and which allows for easy bit replacement. One solution has been to have a two piece means for attaching the bit, a base piece and a holder piece. A base piece is securely attached to the mining apparatus, such as for example by welding such a base piece to the rotating drum of a continuous miner. Such a base piece may have a socket therein adapted to receive a holder piece. The holder piece is adapted to receive the bit and to be readily replaceable in the base piece as this holder piece is also subject to wear during normal operation. Such a system of bit, holder, and base allow each component to be replaced independently and with an ease of replacement generally proportionate to its expected life.
In the past two approaches to solving the problems of durability and replaceability in cutter bit attachment have been developed. The first approach has been to use large steel mounting blocks and heavy lug type bit holders. The heavy lug is either pinned or bolted to the block to secure the two portions. While this type of lug and block arrangement solve some of the problems inherent in bit attachment, the basic approach is strength through sheer physical size. This solution naturally results in a large block and lug with corresponding high manufacturing costs and unnecessary weight. Since this approach has placed paramount emphasis on size to resist the primary cutting forces it has not addressed itself to the problem of movement between the lug and mounting block. Such movement between a bit holder and a mounting base can cause unnecessary wear life of the holder and mounting base. This wear results in shortened life of the mounting base and the mining machine driving component, such as for example the cutting drum on a continuous miner. U.S. Pat. No. Re. 28,310 and U.S. Pat. No. 3,697,137 are representative of this large lug and block approach to the problem.
An alternative approach to the problem has been to use a mounting base or block having a central socket. A bit holder having a shank portion is inserted into the socket and clamped in place. This socket type approach generally results in a smaller more efficient bit holder and base assembly. U.S. Pat. Nos. 3,498,677 and 3,992,061 are indicative of this approach. Such socket type designs have concentrated on supporting the bit and bit holder to resist the primary cutting forces by urging the shank portion of the bit holder rearward and downward in the sockets. By use of the holder shank and efficiently designed support surfaces on the bit holder this approach has resulted in a smaller assembly that adequately resists the primary cutting forces. The small size of the socket type bit holder creates another consideration in that the moment arm from the load bearing surfaces to the axis of the shank has been reduced. This reduction in moment arm lessens the effectiveness of the holder and base combination to resist rotation of the holder about the axis of the shank. Rotational torques about the shank axis result in unnecessary wear to both the holder and the bit. This invention provides for resisting these rotational torques and retarding rotational movement of the holder about the central axis of the socket.
Because the base should have a long life, it is usually welded to the mining machine, drum, or chain; and this securing attachment represents little real problems. This invention relates to the attachments of the bit to the holder, and the holder to the base. One such attachment has been the use of a base having a socket therein and a holder having a shank which engages such socket. A clamping means such as for example a bolt is used to secure the shank in the socket. This invention relates to the means by which the bit transmits forces to the holder and the holder in turn transmits forces to the base. Such a socket type base and holder are disclosed in U.S. Pat. No. 3,992,061.
During mining operation the cutter bit is subjected to extremely large mechanical forces of varying magnitude and direction. One such mining operation for example is the use of cutter mounted on a rotating cutter drum of a continuous miner for coal mining. The first of such forces or cutting force on a single cutter bit is generally tangential and varies during a single revolution of the drum from a high cutting force during contact with the coal, to low or negative force when not contacting the coal. The magnitude of this cutting force also varies within a single cutting stroke as the consistency of the coal and its structural arrangement varies.
In addition to the actual cutting force it is necessary to apply a normal pressure against the coal to force the cutter into contact with the face. The second or normal force is quite high during sumping or shearing operations. Again the normal force varies during the cutting stroke depending up the consistency and structure of the coal. This normal force on the bit is generally direct toward the axis of the drum during the cutting stroke. On the second half of drum rotation, after completing the cutting stroke when the bit is not contacting the coal, an outward force and a tangential centrifugal force results due to the drum rotation and the bit support structures.
A third significant force present during cutting operations is a side or lateral thrust generally perpendicular to the plane of the cutting and normal forces described above. This side thrust results from inperfections in the coal, misalignment of the cutter, or side movement of the driving mechanism such as for example cutting clearance with the continuous miner drum. This side force tends to produce rotation of the holder in the socket resulting in assembly failure or poor cutting.
While other forces may be exerted on the cutter assembly during operation such other forces can usually be resolved into components which act similar to the three forces described above. Because of the magnitude and rate of change of these forces it is of prime importance that holder assemblies be capable of supporting the force load and retarding the oscillations and movement associated with such quickly changing loads. When the holder to base connection is given any degree of movement, about the socket axis such movement results in unnecessary wear and premature failure. This invention relates to a cutter assembly that provides support, inhibits movement, and allows for easy removal and replacement of its component parts.
As more support surfaces are added to the bit holder, it becomes increasingly difficult to achieve a proper seating between the bit holder and the bit base. This improper seating is due to a number of causes including manufacturing tolerances, normal wear on the base, and the presence of foreign matter such as, for example, coal dust during a field replacement of a bit holder. The U.S. Pat. No. 3,992,061 explains and solves some of the seating problems. But, the problem existed of how to add load support surfaces which support against the lateral forces in a manner which does not impair the proper seating of the bit holder and base.
Previous holder and base assembly designs concentrated on the ability to sustain the primary cutting force and the normal force. Such designs resulted in holders which through their material bulk restrained some of the side forces; but when such designs are subjected to severe duty having high side loadings the life expectancy is greatly reduced.
While cutter bits may take many shapes, they generally have a conical tip portion that actually engages the material to be mined, and a body portion that is attached to the holder. This invention relates to securing the body portion of the bit to the holder. One such type of bit is a bit having a generally conical tip portion and a shank portion for insertion into a bore in the holder. This invention relates to a means for supporting the bit within the holder.
It is well known that a conical type bit which is relatively free to rotate about the axis of the shank will have a longer bit life and be more effective for cutting operations. For this reason it is desirable to have a bit shank of generally circular cross section which freely rotates within the bore of the bit holder. The problem has been to adequately support the bit against the forces present in the cutting operation and still permit the bit to freely rotate. One method for holding the bit and allowing rotation has included a sloping shoulder on the forward portion of the bit shank which bears upon a recessed surface on the front of the holder. Such a forward shoulder is shown in U.S. Pat. No. 3,499,685. This type of shoulder support requires that the shoulder area transmit the complete axial component of the cutting forces. Such large forces on a small limited area result in wear due to friction.
Another method of supporting the bit includes a rear surface on the bit shank which engages a surface on a portion of the holder. This rear support method simplifies the bit and holder engagement but the bearing surface is limited to the cross sectional area of the bit shaft or shank. U.S. Pat. Nos. 3,397,012 and 3,554,605 show this method of rear bit contact.